This invention relates to a composition useful for preparing a semi-permeable cellulose ester membrane useful for gas separation. This invention further relates to a process utilizing said composition to prepare a semi-permeable cellulose ester membrane useful for gas separation.
Cellulose ester membranes have long been used to isolate or recover gases such as hydrogen, helium, oxygen, nitrogen, carbon dioxide, methane, light hydrocarbons, hydrogen sulfide, and water vapor from gas mixtures. Particular separations of interest include the separation of hydrogen or helium from natural gas and the separation of nitrogen or carbon dioxide from light hydrocarbons or other crude oil components as part of the tertiary oil recovery process. The dehydration of natural gas is another gas separation of interest. Other applications include the recovery of an enriched oxygen stream from air for use in enhanced combustion processes. Alternately, an enriched nitrogen stream may be obtained from air for use as an inert atmosphere over flammable fluids or for food storage or metal treating.
Typically, cellulose ester membranes are prepared by extruding, molding, or casting the membranes from blends containing a polymer, a solvent, and an optional non-solvent. Solvents are compounds in which the polymer substantially dissolves at the membrane fabrication temperature. Non-solvents are compounds in which the polymer is substantially insoluble at the membrane fabrication temperature. Solvents which have been used to prepare cellulose ester membranes include sulfolane, dimethylformamide, N-methylpyrrolidone, and acetone. Non-solvents used to prepare cellulose ester membranes include methanol, propanol, water, and maleic acid. Polyethylene glycol has been used as a non-solvent for cellulose triacetate. Such solvents and non-solvents are environmentally harmful and deleterious and once removed from the membranes during subsequent fabrication present problems of disposal or extensive repurification before reuse. Such solvents and non-solvents also present problems with respect to exposure of personnel to harmful and deleterious vapors and liquids during the membrane fabrication process.
What is needed are extrusion, molding, or casting compositions for cellulose esters in which the solvents and non-solvents are not harmful or deleterious.